1. Field of the Invention
The present invention relates to a direct fuel injection ignition engine in which a cavity is formed on a top of a piston and a fuel injector which injects a fuel directly into an engine combustion chamber is disposed on a peripheral portion of an ignition chamber.
2. Related Art
Conventionally, as shown in Japanese Patent Un-examined Application Publication No.7-286520, a direct fuel injection ignition engine has been known that an ignition plug is disposed in a middle portion of a combustion chamber and that an injector is disposed at a peripheral portion of the combustion chamber so as to inject a fuel directly into the combustion chamber. In this type of engine, the injector is disposed at the peripheral portion of the combustion chamber so that the fuel is injected obliquely downwardly toward the piston top. When the fuel is injected in a compression stroke, the fuel reflected on the top surface of the piston is provided around the ignition plug to accomplish a stratification combustion and when the fuel injection is made during an intake stroke, an air fuel combustion gas is dispersed in the whole combustion chamber to accomplish a uniform combustion.
A control has been done in a manner that a fuel injection pattern is changed in accordance with an operating condition. For example, a stratified combustion is made on a low engine load and low engine speed condition through a compression stroke injection in which the fuel injection is made during the engine compression stroke. The fuel injection is made during the intake stroke when the operating condition is in a high engine load and/or a high engine speed condition. In addition, an air fuel ratio is controlled in accordance with the fuel injection pattern control. For example, the air fuel ratio is controlled to provide a lean air fuel mixture when the stratified combustion is executed. The air fuel ratio is controlled to provide a lean air fuel mixture in a lower engine load condition in the case where the uniform combustion is established while the air fuel ratio is controlled to provide a rich air fuel mixture in a high engine load condition.
It is proposed that a cavity is formed on the top of the piston in order to facilitate the stratification so that the fuel injected in the compression stroke is introduced to the ignition plug by way of the cavity.
However, in the conventional engine, there has been a problem that when an inclination angle of the injector to a plane perpendicular to an axis of a cylinder bore is too small, a significant amount of the fuel injected from the injector is not trapped in the cavity and dispersed outside of the cavity of the top surface of the piston resulting in a deterioration of a fuel combustion efficiency and combustion stability. In addition, when the uniform combustion is executed in the intake stroke injection, the amount of the fuel attached to a cylinder wall surface is increased to increase the amount of hydrocarbon emission and incur a dilution of a lubrication oil.
If the inclination angle of the injector is increased, the above problem is eliminated. However, if the inclination angle is increased, it would be difficult to form properly intake ports due to the interference with the injector. Thus, the increase of the inclination angle of the injector is impossible due to the above problem in layout.
In order to solve the above problem, forming a tumble in the combustion chamber would be effective. The tumble would orient the atomization from the injector downward so that the trapping of the injected fuel by the cavity is facilitated in the course of the stratified combustion. In addition, the amount of the fuel attached to the wall surface of the cylinder can be reduced. However, it is not enough for the tumble to introduce the trapped air fuel mixture to the ignition plug so that the air fuel ratio of the air fuel mixture around the ignition plug becomes unstable. In addition, the tumble component tends to be reduced and collapsed. As a result, the air fuel mixture cannot be properly dispersed in the stratified combustion of a middle engine load condition in which the fuel injection amount is fairly large, and resulting in a deterioration of the fuel consumption efficiency.
Alternatively, it would be proposed that an intake system is constituted to facilitate a swirl. However, the sole swirl formation may produce an inversed stratified condition in which the fuel air fuel mixture is concentrated around the peripheral portion of the combustion chamber rather than around the ignition plug. Thus, the combustion performance cannot be improved.